Attenuation of cold-induced apoptosis by exogenous melatonin in carrot suspension cells: the possible involvement of polyamines.

Pretreatment with 43 nM (10 ng/mL) to 86 nM melatonin for 5 days significantly attenuated cold-induced apoptosis in carrot suspension cells (Daucus carota L.) as evidenced by the TUNEL procedure, DNA fragmentation and the morphological changes revealed by electronic microscopy observations. The antiapoptotic effect of melatonin was initially thought to be a result of its antioxidant actions. In our study, however, reactive oxygen species (ROS) generation remained unaffected by melatonin treatment, suggesting that melatonin plays its protective role not related to its direct ROS scavenger. At the same time, notable increases in putrescine and spermidine levels were observed in melatonin-treated cells, which may be responsible for the alleviation of the cold-induced apoptosis. The possible involvement of polyamines in the antiapoptotic effect of melatonin was further confirmed by the inhibitory effect of exogenous polyamines on apoptosis as displayed by the DNA laddering assay.